1. Field of the Invention
The present invention relates to an optical transmission line which is formed by connecting a plurality of optical fibers.
2. Description of the Background Art
In an optical transmission system, signal light which is transmitted from an optical transmitter travels through an optical transmission line comprised of optical fibers and is received by an optical receiver. Between the optical transmitter and the optical receiver, there is provided an optical repeater which includes an optical amplifier for optically amplifying signal light and an optical branching device, etc. For complying with an expansion of relay interval and an increase in the number of branches in such an optical transmission system, the power of signal light which enters into the optical transmission line must be enhanced. However, it has been a problem that even if the incident power of signal light is enhanced, the optical power to be propagated through an optical fiber is limited because of back-scattering light that is caused by stimulated Brillouin scattering (SBS) while the signal light travels through the optical fiber. The SBS is one of nonlinear phenomena.
The incident optical power level in which back-scattering light occurs conspicuously to the extent that cannot be ignored is called as a SBS threshold. Theoretically, a SBS threshold Pth is expressed by formulas 1a and 1b:
                              P          th                =                  21          ⁢                                                    K                0                            ⁢                              A                eff                                                    G              ⁢                                                          ⁢                              (                                  v                  max                                )                                                                        (                  1          ⁢          a                )                                          G          ⁢                                          ⁢                      (            v            )                          =                              ∫            0            L                    ⁢                                                    g                B                            ⁡                              (                                  v                  ⁢                                      ,                                    ⁢                  z                                )                                      ⁢                                                  ⁢            exp            ⁢                                                  ⁢                          (                              -                az                            )                        ⁢                          ⅆ              z                                                          (                  1          ⁢          b                )            where K0 is a constant depending on polarization, Aeff is an effective core area of an optical fiber, G(ν) is an effective Brillouin gain coefficient at a Brillouin frequency shift ν, νmax is a Brillouin frequency shift in which G(ν) becomes maximum, gB(ν, z) is a Brillouin gain coefficient at a position z in the longitudinal direction, α is a loss coefficient of an optical fiber, and L is a fiber length.
A known method for suppressing SBS is a method in which a plurality of optical fibers that differ from each other in terms of Brillouin frequency shift are connected in cascade connection. An optical transmission line described in T. Sugie, et al, J. Lightwave Techn., Vol. 9, No. 9, pp. 1178–1186 (1991) (Document 1) is formed by connecting four optical fibers whose Brillouin frequency shift are 10.92 GHz, 10.90 GHz, 10.80 GHz, and 10.90 GHz, respectively. An optical transmission line described in X. P. Mao, et al., IEEE Photonics Techn. Lett., Vol. 4, No. 1, pp. 66–69 (1992) (Document 2) is formed by connecting, in cascade connection, 10 or more optical fibers which differ from each other in terms of Brillouin frequency shift.
Conventional optical transmission lines are such that SBS can be suppressed whereas no consideration is given to other transmission characteristics (e.g., low OH absorption loss, conformity to international standards, low OTDR measurement error, etc.). In other words, the Brillouin frequency shift of an optical fiber can be adjusted by changing the refractive index profile of the optical fiber, but the other transmission characteristics also change at the same time.